With the progress of society and abundance of people lives, the public pays more and more attention to their health. The public is interested in scientific research and nutritional supplement of polyunsaturated fatty acids (PUFA) such as fish oil.
PUFA is an important basic substance for body metabolism, especially for infant brain development. PUFA is a component of cell membrane, and plays a role on physiological functions such as maintaining cell membrane fluidity, reducing cholesterol, improving insulin sensitivity, decreasing blood glucose, decreasing cholesteremia, and reducing fat accumulation, treatment of diabetes, lowering blood pressure, regulating heart rate, regulating thrombosis and treatment of arthritis. However, PUFA cannot be synthesized by the body itself. PUFA must be obtained from diet.
There are various kinds of PUFA including ω-3 PUFA, ω-6 PUFA, ω-9 PUFA, and other kinds of conjugated linoleic acid, such as α-Linolenic acid (ALA), eicosapentaenoic ester (EPA), docosahexaenoic acid (DHA), docosapentaenoic acid (DPA), linoleic acid (LA), conjugated linoleic acid (CLA), γ-linolenic acid (GLA), arachidonic acid (AA) and so on. Wherein EPA and DHA representing ω-3 PUFA are known and acceptable to the public. and obviously improve human thinking and enhance memory. Their molecular structures of polyunsaturated fatty acids are as follows.

Polyunsaturated fatty acids mainly derive from algae extract and marine oil. One of important sources comes from fish oil.
At present, glyceride type polyunsaturated fatty acids occupy a large proportion in main application forms of polyunsaturated fatty acid products or materials in industry, because glyceride type polyunsaturated fatty acids are more stable relative to free type polyunsaturated fatty acids and ethyl ester type polyunsaturated fatty acids. Free type polyunsaturated fatty acids are easily oxidized, and their bioavailability of ethyl ester type polyunsaturated fatty acids and methyl ester type polyunsaturated fatty acids in the body is lower and consequently there would be some safety problems. Moreover the bioavailability of the glyceride type polyunsaturated fatty acid in the body is higher than other types of polyunsaturated fatty acids, and the glyceride type polyunsaturated fatty acids have also been proved to have better security.
It would be very difficult to obtain glyceride type polyunsaturated fatty acids by direct distillation or other methods because of higher boiling point of glyceride type polyunsaturated fatty acids. At present, glyceride type polyunsaturated fatty acids in industry are mainly obtained by a direct esterification or transesterification of polyunsaturated fatty acids such as free-type or ethyl ester-type or methyl ester-type with glycerol or glyceride.
The prior art mainly includes that a polyunsaturated fatty acid material performs a esterification or transesterification with glycerol or glyceride by adding lipase catalyst or adding sodium hydroxide or potassium hydroxide and other alkali catalyst or adding zinc powder for protection without catalyst at high temperature, and then obtains glyceride type polyunsaturated fatty acid products after subsequent processing. Difficulties of the process mainly focus on poor reaction activity of glycerides, and raw materials or products are easily oxidized and destroyed because of many double bonds in their molecular structures.
The lipase catalytic process has the following drawbacks: 1) the reaction cost is higher because of higher cost of enzyme; 2) the final product has some limitation because of selectivity of enzyme catalysis, for example, an enzyme catalysis is effective for 1,3-position hydroxyl of glycerol but invalid for 2-position hydroxyl of glycerol; 3) the lipase catalytic process has a longer reaction time generally and consequently results in certain difficulties in large scale industrial production; 4) enzymes in the reaction are easily affected by raw materials or environmental factors and consequently results in inactivation of enzymes. The prior process has higher demand for raw materials and environment, and the cost of enzyme catalytic process is also higher. So the prior process has certain limitation in large-scale production in industry.
The chemical catalytic process of preparing glyceride type polyunsaturated fatty acids by adding sodium hydroxide or potassium hydroxide and other strong alkaline catalyst also has some deficiencies as follows: 1) sodium hydroxide or potassium hydroxide or other strong alkaline has certain destructive effects on raw materials of polyunsaturated fatty acids; 2) the prior art has larger amount of catalyst, higher reaction temperature, less complete reaction, lower reaction yield and longer reaction time, because of poorer catalytic activity of sodium hydroxide or potassium hydroxide. So it would lead to certain destructive effects on raw materials of polyunsaturated fatty acids, darker product color. So the final product quality is poor.
Non-catalytic process of preparing glyceride type polyunsaturated fatty acids is mainly limited to necessarily use free type polyunsaturated fatty acids as raw materials. It is necessary to hydrolyze a variety of ester types of polyunsaturated fatty acids to obtain free type polyunsaturated fatty acids firstly, and then perform esterification or transesterification. So the process is more complicated. At the same time, the prior process has low reaction degree, low reaction yield, longer reaction time, higher reaction temperature, because of no catalyst in the reaction. So the final product quality is poor.
At present, some literatures disclose processes of preparing glyceride type polyunsaturated fatty acid by adding polyunsaturated fatty acid materials.
Patent CN102277237 mainly describes to perform a saponification of ethyl ester-type polyunsaturated fatty acids, and then acidify and wash, and recycle solvent to obtain free-type polyunsaturated fatty acids, and then add zinc powder, sodium hydroxide or potassium hydroxide as catalyst to free type polyunsaturated fatty acids to react together at 180˜225° C., finally go through a quenching reaction by rapid cooling, and then extract, wash, drying and concentrate in turn, to obtain glyceride type polyunsaturated fatty acids. The process is more complicated, poor product quality, 80% of product yield.
Patent CN103242969 describes to directly react ethyl ester-type polyunsaturated fatty acids with glycerol to obtain glyceride type polyunsaturated fatty acids by adding sodium hydroxide or potassium hydroxide as catalyst. The process needs high reaction temperature due to poor catalytic activity of the catalyst. So the product quality is poor, and the reaction yield is also low about 70%.
Patents JP200213359, CN101161819, CN101255380, CN10176044, CN1884564, CN101818176, CN103436563, CN102028711 describes a process of reacting ester type polyunsaturated fatty acids or free type polyunsaturated fatty acids with glycerol or glyceride by adding lipase catalyst to prepare glyceride polyunsaturated fatty acids. The process has some deficiencies such as higher cost of lipase, selective position of glycerol esterification, poisonous lipase, complicated process, higher cost.
In general, the prior processes of glyceride type polyunsaturated fatty acids have one or more deficiencies such as 1) complicated process and higher cost; 2) low reaction degree and longer reaction time and lower yield; 3) easily damaged of raw materials and poor product quality due to high temperature or strong acidity or basicity.
In general, glyceride type polyunsaturated fatty acid products are mainly obtained by reacting polyunsaturated fatty acid materials with glycerol or glyceride. But it would result in difficulty of direct reaction with polyunsaturated fatty acid materials because glycerol or glyceride with three reaction sites has structural steric hindrance and low reactivity of glycerol or glyceride. Besides, strong acids as necessary catalyst would directly cause destruction of raw materials, but weak acids or weak bases as catalyst could not catalyze such a reaction because of poor catalyst activity of weak acids or weak bases.
Strong basic catalysts have relatively better catalytic activity for such a reaction, and consequently are widely used in preparation of lower fatty acid esters. Such a process has good effects, small destruction on raw materials and better product quality. However, as for the preparation of higher fatty acid esters, the alkalinity of catalysts could have a certain degree of damage on raw materials especially lower stable raw materials. Wherein, the activity of sodium hydroxide or potassium hydroxide is poor. So it would need a big feeding amount of catalyst. But it would result in incomplete reaction and low yield in the process of preparing polyunsaturated fatty acid. And the alkalinity of catalysts has more serious effects on polyunsaturated fatty acid materials and produces poor quality products. The alkalinity of sodium or potassium alkoxides or its solutions as catalyst would have destructive to raw materials and products and obtain poor quality products, although sodium or potassium alkoxides or its solutions with smaller amount have better catalytic activity.